Abstract

Carbonate reservoirs often exhibit complex pore networks and various scales of heterogeneity associated with stratigraphic cyclicity, facies distribution, and diagenesis. In addition, variability also exists within distinct rock fabrics at the inter-well scale. Data from lateral transects through dolomitized carbonates of the Mississippian Madison Formation in north and central Wyoming (Figures 1 and 2) exhibit three scales of variability in the lateral distributions of porosity, permeability, and trace-element concentrations. Random variations at ≤30-cm spacings account for 50-70% of the total variability. The balance of the variability occurs in short- and long-range periodic patterns that range from 1.5 to >48 m and are resolved only by high-resolution sampling of ~150-m-long lateral transects. This variability is observed in both dolowackestone and dolograinstone facies (Figure 3). Possible origins for these patterns are: (1) inheritance from the depositional precursor, (2) formation by self-organizing processes during dolomitization, or (3) overprinting by late recrystallization and/or dolomite cementation.

Detailed petrophysical models of the dolomite facies and multiphase waterflood simulations explore the effects of this heterogeneity on reservoir performance through several model scenarios. Models with greater short-scale continuity of petrophysical properties have higher degrees of large-scale fingering, higher sweep efficiency, and shorter breakthrough times. The reservoir performance of the dolowackestone differs from he dolograinstone for those models that exhibit a specific range of short-scale heterogeneity. In general, the dolowackestone has a higher degree of both small- and large-scale fingering, lower sweep efficiency, and longer break-through time compared to the dolograinstone.

Intra-facies scale variability is significant in regard to reservoir performance and is often difficult or impossible to determine from typical subsurface data sets. Information from outcrop analogs is necessary to create conceptual 3-D geologic models and to begin to quantify inter-well heterogeneity within dolomite reservoirs. The reservoir-scale patterns in the lateral distributions of petrophysical and geochemical properties have heretofore been unrecognized and addressing their origin and meaning(s) represents a new approach to the study of dolomites and dolomite reservoirs. The recognition of periodic patterns in dolostones and understanding their origin has significant implications.

The periodic patterns in porosity and permeability can affect pore connectivity and fluid flow. Thus those periodic patterns should be understood so that they can be incorporated into dolomite reservoir models and models of contaminant transport from matrix to conduits in dolomite aquifers.

If the periodic patterns are self-organizing phenomena, then they can be used to interpret ancient dolomitizing systems in ways previously not realized.